The invention provides a blowout preventer (hereinafter “BOP”) with improved seal components for the rams.
One example of a prior art BOP is disclosed in U.S. Pat. No. 5,765,813 to Lam et al., issued Jun. 16, 1998. This type of BOP is commonly used in connection with pumping wells. With such wells, a sucker rod string is reciprocated or rotated to drive a downhole pump, which lifts the produced fluid to surface through a tubing string. The BOP is equipped with rams which can be advanced horizontally to seal around the vertical polish rod of the rod string, to prevent the upward escape of fluid. Alternatively, if the rod string is out of the well, the inner ends of the rams can be pressed together to cause closure of the wellhead assembly fluid passageway.
More particularly, the prior art BOP includes a cross-shaped housing forming a vertical bore and a pair of coaxial, horizontal ram bores intersecting the vertical bore from each side. The BOP is commonly positioned in the wellhead assembly between the tubing head and flow tee. In such an embodiment, the BOP vertical bore forms part of the wellhead assembly fluid passageway.
Within the BOP, a pair of rams is positioned in the horizontal ram bores. Means, such as screw jacks or hydraulic actuators, are provided at the outer ends of the ram bores, for advancing or retracting the rams into or out of the vertical bore, in order to close or open the vertical well bore.
Each ram comprises a generally cylindrical body. The body comprises a steel core having an outer full diameter portion and a reduced diameter inner portion. The ram core inner portion is covered with and bonded to a layer of an elastomer, typically nitrile rubber.
The cylindrical ram bores extend into the vertical bore and the bore surfaces combine at their intersection to form sealing areas. When the rams move into the vertical bore, the rubber surfaces of their inner portions seal against the sealing surfaces.
The rubber-coated inner face of each ram is formed to provide a semi-circular, vertically directed groove. Thus, when the polish rod of the rod string is present in the vertical bore, the ram ends encircle and press against it, to form a seal of the vertical bore. When the polish rod is not in the vertical bore, the ram ends compress together to form a solid block. In both cases, the circumferential seals of the ram side surfaces with the sealing areas and the end face seals combine to close the vertical bore and contain pressurized fluids seeking to pass therethrough.
One major problem associated with production BOP's is that the pressure acting from below on the closed rams may extrude the side rubber upwardly so that the circumferential seal with the sealing areas is lost. Another problem is that the end rubber bonded to the vertical end faces of the ram cores may tear loose from the core when high pressure is exerted from below. In severe conditions such as injecting chemicals to close off a well, the rubber degrades quickly, causing the seals to fail.
In U.S. Pat. No. 5,765,813, the side surface of each ram core inner portion is formed to provide ribs extending outwardly therefrom and extending longitudinally thereof. The ribs are located so as to underlie those portions of the elastomer layer which seal against the sealing areas. The elastomer layer is thinned where it passes over the ribs, to preserve the cylindrical shape of the ram's inner portion. The ribs function to reinforce the elastomer layer against upward extrusion. Comparative pressure tests have shown that a ribbed ram was able to contain several times the pressure that a non-ribbed ram could contain, before the seal was lost.
In accordance with a preferred feature of U.S. Pat. No. 5,765,813, the vertical end face of the core inner portion is indented, for example with a plurality of spaced apart transverse grooves or dimples or a single cavity, so that the base of the elastomer layer projects into these indentations. As a result, increased face surface area is provided to bond with the elastomer. In addition, the elastomer external of the indentations must shear from the elastomer in the indentations, before failure occurs and the elastomer separates from the steel. In this way, the resistance to failure of the elastomer/steel bond at the ram end faces is improved.
The device of U.S. Pat. No. 5,765,813 improves the ability of the BOP ram to seal against the high pressure from below without losing the side seal, and better prevent against the rubber being torn loose from the ram.
Canadian Patent Application No. 2,260,655, published Aug. 2, 2000, and owned by Stream-Flo Industries Ltd., describes a ram type BOP for high temperature applications. The BOP incorporates a generally L-shaped seal element formed of a graphite or asbestos seal material, which is sandwiched between a bottom L-shaped steel retainer plate and a semi-cylindrically shaped steel top retainer plate. This design of BOP ram is well suited for brittle seal materials such as graphite and asbestos, which are used when steaming of the well may occur.
Other prior art BOPs using L-shaped seal elements or similar devices are described in the patent literature, but all utilize rubber-type sealing material sandwiched between steel body components, for instance, see U.S. Pat. No. 1,963,683 issued Jun. 19, 1934 to Shaffer; U.S. Pat. No. 3,023,994 issued Mar. 6, 1962 to Gibson; U.S. Pat. No. 3,904,212, issued Sep. 9, 1975 to Pugh et al.; and U.S. Pat. No. 4,416,441 issued Nov. 22, 1983 to Van Winkle.
None of the above patents adequately provide BOPs for use in harsh chemical conditions. Rubber-type seal materials are subject to being degraded by the harsh conditions of some wellheads, for example where chemicals are injected. For instance, in wells in which heavy wax deposits accumulate in the annulus, outlet lines, and anywhere that the production fluid flows, a chemical inhibitor is introduced into the production fluid as a means to break down the deposits. While the inhibitor is effective in clearing the lines, it has the major disadvantage of acting as a dissolving agent on the O-ring seals, rubber ram inserts and any elastomeric materials found inside the wells, causing these seals to fail prematurely.
Thermoplastic materials having superior chemical resistance are available, but have different compressibility and elastic properties than elastomeric sealing materials such as nitrile rubbers. Thus, the prior art BOP devices are not generally amenable to simple substitution of thermoplastics for the rubber sealing components.
Some patent literature shows BOPs which make use of thermoplastic materials, but these simply provide U-shaped thermoplastic seal inserts in the front face of a complicated BOP ram design, see for example U.S. Pat. No. 4,398,729, issued Aug. 16, 1983 to Bishop et al.; U.S. Pat. No. 5,127,623 issued Jul. 7, 1992 to McDugle; and U.S. Pat. No. 5,833,208 issued Nov. 10, 1998 to Lee, Jr. These types of BOP devices achieve a seal only by force applied to the seal from the front face against the polish rod.
It is an objective of the invention to provide an improved BOP with seals which, when used in combination with the BOP housing, are able to withstand both high pressure from below, and the environment of harsh chemical, while maintaining the seals over an acceptable time frame.